1. Field of the Invention
The present invention relates to a reflectance estimating method, and more particularly to a reflectance estimating method for estimating a reflectance of an object at an arbitrary displacement angle which displacement angle is an angle representing displacement from the direction of a regular reflection of light from the object.
2. Description of the Related Art
In the field of computer graphics (so-called CG) which enable presentation of an image display with colors on a computer or printing of such image, there are executed an image display of patterns or graphics of various colors and a printing of the displayed color image. In the case of displaying an object as a colored image, the object is presented by luminance of the light through modeling the principle of light reflection, but the color of the object cannot be sufficiently represented by the luminance presentation only of the object, and in such a case, it is difficult to obtain a substantial color presentation (color reproduction) of the object.
In order to obtain a color reproduction exactly representing the appearance of the object under an arbitrary environment, it is necessary to consider not only the reflectance in a normal reflecting direction what is ordinarily employed as the reflectance, but also a reflectance at a displacement angle, which is an angle representing displacement from the normal light reflecting direction of the object (for example spectral steric angle reflectance).
However, a determination of the reflectance for each displacement angle requires an enormous amount of data and is scarcely practical.
For this reason, there is proposed a reflectance measuring method, in which the reflectances are measured corresponding to randomly selected plural displacement angles and the measured data are used to estimate a reflectance corresponding to another displacement angle (cf. JP-A No. 11-230831). This measuring method estimates the reflectance of a displacement angle other than the selected ones, by an interpolation process utilizing colorimetry values obtained in directions of a predetermined number.
However, since the reflectance of an object is not linear nor regular, a large number of samples (displacement angles) have to be selected in order to achieve a highly precise estimation in the prior measuring method. Therefore, in order to estimate an arbitrary color (reflectance) with a certain precision, it is necessary to in advance prepare data measured in a large number of displacement angles and to use such data in the determination of the reflectance, so that the determination of the reflectance has not been possible in an efficient manner.
Also in considering the reflectance at a displacement angle, a light incident angle to the object is often selected at 45°, but a light receiving angle (displacement angle) for measuring the reflectance is not generally defined. It is different depending on the measuring apparatus used or on the measuring method. In this manner, there has not been considered the properness of the light receiving angle (displacement angle) for measuring the reflectance, which is to be used for estimating the reflectance at an arbitrary displacement angle.
For example, there is already known a technology (U.S. Pat. No. 4,479,718) for estimating the reflectance on a solid surface or a metallic coated surface, and this technology utilizes 15°, 45° and 110° as displacement angles. In this technology, however, since the displacement angles determined for a solid or metallic coated surface are empirically handled, an obtained result cannot be ensured for a change in the displacement angle. It is also difficult to estimate a reflectance on an arbitrary object surface.